Field of the Invention
The present invention relates to phase compensation of a voltage regulator.
Background Art
In general, a voltage regulator receives an input voltage supplied to an input terminal thereof and generates a fixed output voltage to an output terminal thereof. The voltage regulator supplies current according to a load to always maintain the output voltage at a constant level.
FIG. 2 is a circuit diagram of a conventional voltage regulator.
A reference voltage circuit 10 generates a reference voltage VREF. Bleeder resistors 11 and 12 divide an output voltage VOUT of an output terminal 3 to generate a feedback voltage VFB. A differential amplifier circuit 13 receives the reference voltage VREF and the feedback voltage VFB through the input terminal thereof An output voltage of the differential amplifier circuit 13 is input to a constant current source 17 and the gate of a PMOS transistor 16 constituting a first source ground amplifier circuit. A resistor 14 and a capacitance 15 form a phase compensation circuit. An output control MOS transistor 25 constituting a second source ground amplifier circuit receives, through the gate thereof, an output voltage of the first source ground amplifier circuit. A load is connected to the output terminal 3 of the voltage regulator.
The operation of the conventional voltage regulator will be described.
If the output voltage VOUT of the output terminal of the voltage regulator decreases, then the feedback voltage VFB decreases. If the feedback voltage VFB decreases below the reference voltage VREF, then the output of the differential amplifier circuit 13 increases and the ON resistance of the PMOS transistor 16 increases. This causes the output voltage of the first source ground amplifier circuit to decrease, so that the ON resistance of the output control MOS transistor 25 decreases. Hence, the output voltage VOUT of the output terminal of the voltage regulator increases.
Meanwhile, when the output voltage VOUT of the output terminal of the voltage regulator increases, the voltage regulator performs an operation that is opposite from the above, so that the output voltage VOUT of the output terminal of the voltage regulator decreases. Thus, the voltage regulator works to make the feedback voltage VFB and the reference voltage VREF equal so as to generate the constant output voltage VOUT.
The voltage regulator is required to have a wider frequency band in which feedback amplification is possible so as to improve its transient response characteristics. The conventional voltage regulator is configured to have a voltage 3-stage amplifier circuit to permit an expanded frequency band that allows feedback amplification to be accomplished with a relatively small consumption current, thus improving the transient response characteristics. However, the configuration that involves the voltage 3-stage amplifier circuit tends to cause an electric signal, which has gone round a feedback loop once, to develop a phase lag of 180 degrees or more. This may lead to an unstable operation of the voltage regulator and result in an oscillation in the worst case.
The conventional voltage regulator therefore additionally includes a phase compensation circuit composed of the resistor 14 and the capacitance 15 in order to compensate for the foregoing phase lag. More specifically, the resistor 14 and the capacitance 15 resets the phase at a zero point thereby to prevent an oscillation (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-62374
However, according to the conventional voltage regulator, in the case where the gate capacitance of an output control MOS transistor is large, a phase compensation capacitance that is equivalent to or larger than the foregoing gate capacitance is required to accomplish pole separation. This inevitably increases a chip area with a resultant higher cost.